1. Field of the Invention
This invention relates to a diaphragm drive device for driving the diaphragm blades by a resin-molded magnet rotor.
2. Description of the Related Art
In the past, as the motor for driving the diaphragm or shutter blades in the camera, as shown in Japanese Utility Model Publication No. SHO 53-45632, there is known the structure of the diaphragm drive device comprising an annular rotor concentrically arranged on the outer periphery of tubular body, an annular stator concentrically arranged as directed toward one end surface of this rotor, a rotatable frame fixed to the rotor, rollers supporting the rotatable frame to rotate freely, these parts being the main parts, and the rotor having the form of a flat plate ring-shaped permanent magnet plate made of ferrite.
Also, in the conventional interchangeable lens barrel, the opening and closing of the diaphragm blades has been controlled in such a manner that the activating force resulting from the exposure control operation from the camera side is transmitted to the lens side by a lever mechanism between the lens and the camera body, thereby the diaphragm blades are opened and closed by the rotating operation of the lever.
However, in recent years, the use of electronics in the control of the camera system has led to proposals for attempting the control of the diaphragm blades within the interchangeable lens barrel by means of a motor as in U.S. Pat. No. 4,534,624 (corresponding to the Japanese Laid-Open Patent Applications Nos. SHO 58-16208 and SHO 58-10706). Because the opening and closing operation of the diaphragm blades is controlled stepwise in favor of the relative aperture, for the drive accuracy is improved by employing the pulse drive control method combination with the digital control method for the exposure of the camera, it is desirable to make the drive control by the stepping motor. Therefore, a proposal of controlling the diaphragm by the rotor of the stepping motor is also shown in U.S. Pat. No. 4,472,039 (corresponding to Japanese Laid-Open Patent Application No. SHO 58-17428.
The use of the stepping motor in the drive control of the diaphragm blades is advantageous from the above-described reason, and approaches one step ahead toward a perfect electronic system between the lens and the camera body. Yet, a number of problems are left unsolved.
One of them is that the incorporation of the stepping motor in the lens barrel, particularly interchangeable one, calls for a great increase in the size and weight of the barrel. Being not limited to the stepping motor, the motors generally have many constituent parts such as the permanent magnet, coils and yokes. Moreover, the main ones of the constituent parts are all made of metallic materials, and have large specific gravities, contributing to a very large proportion of the overall weight of the barrel.
The second problem lies in the rotor of the motor. In case when the rotor is drivingly connected either directly or indirectly to the diaphragm blades and the opening and closing action of the diaphragm blades is taken by the stepwise drive of the rotor, if the rotor itself is very heavy, the required rotating torque is necessarily large. For this reason, the consumption of electrical power is increased largely.
Also, if the weight of the rotor is large, another problem arises in the point of inertial moment. In more detail, when shooting the camera with the interchangeable lens attached thereto, the object brightness is measured with the diaphragm blades at full open aperture to derive an exposure value, or the so-called open aperture light metering is carried out.
Therefore, the diaphragm blades must be maintained in the full open aperture state always before the light metering. However, upon consideration of the shooting situation of the camera, particularly the single lens reflex camera, it sometimes happens for the photographer to catch an instantaneous shutter chance by quickly moving the camera in pursuit of the situation of the object. In such a case, if the inertial moment of the rotor within the barrel, the rotor will be caused to rotate, accidentally closing down the diaphragm from the full open aperture position. Thus, it becomes impossible to assure that the diaphragm is fully opened when the light metering is carried out.
The third problem also arises from the rotor. As shown in the above-cited Japanese Utility Model Publication No. SHO 53-45632, the rotor is made of magnetic material and formed to a ring shape with a plurality of pairs of N and S poles in a common circle with the center at the optical axis. Therefore, the magnetic flux is produced from both surfaces of the ring. In case when the diaphragm blades are made of metal or magnetizable material, they are influenced by that magnetic flux and, in turn, affect the accuracy of exposure control.